In recent years, the amount of heat generated by electronic devices such as a semiconductor device has been increasing. Take, for example, a CPU for personal computers, the amount of power consumption has increased at twice the rate in the past five years (Furukawa Review No. 106, http://www.furukawa.co.jp/jiho/fj016/fj106—01.pdf (Non-Patent Document 1)), accompanied by increases in the amount of generated heat.
In order to dissipate heat from such electronic devices, a heat dissipator such as a heatsink is generally used. When a heat dissipator is used for cooling, the thermal properties of the material for the heat dissipator greatly affect the cooling performance.
The first thermal property to which attention should be paid is the thermal conductivity, which is preferably high. A high thermal conductivity allows heat to be spread all over the heat dissipator, enabling the heat to be efficiently dissipated to the atmosphere.
The second thermal property to which attention should be paid is the thermal expansion coefficient, which is preferably equal to that of the material of the heat-generating device to be cooled. The heat from the heat-generating device is transmitted to the heat dissipator via contact; if there is a difference between the thermal expansion coefficients of the two, an ideal contact cannot be maintained, which hinders the normal heat transfer.
It is necessary that the heat dissipator material should meet the above requirements.
Aluminum and copper have typically been used as materials for heat dissipators. Aluminum and copper have thermal conductivities of about 200 W/(mK) and about 400 W/(mK), respectively, which are higher than those of general materials (iron: 84 W/(mK), stainless steel: 14 W/(mK), glass: 1 W/(mK), and resin: 1 W/(mK) or lower), while being inexpensive and having excellent workability. A material such as aluminum nitride (thermal conductivity: 250 W/(mK) or lower) or diamond (thermal conductivity: 800 to 2000 W/(mK)) is also used in a portion of a heat dissipator that requires insulating properties or an even higher thermal conductivity; however, the use of such expensive materials is not common.
Moreover, aluminum and copper have high thermal expansion coefficients (23 ppm/K and 17 ppm/K, respectively (both at room temperature (RT) to 100° C.)). On the other hand, silicon, which is a semiconductor material, has a low thermal expansion coefficient (2.6 ppm/K (RT to 100° C.)). For this reason, when aluminum or copper is brought into contact with silicon to allow heat dissipation, there is a difference between the thermal expansion coefficients of the two materials. Therefore, when aluminum or copper is used in a heat dissipator, a grease or the like is used in the contact portion to maintain the contact between the two materials; however, a grease has a thermal conductivity of about 1 W/(mK), which is the same as that of resin, and therefore becomes a large thermal resistance.
Metal-based carbon fiber composite materials have recently attracted attention as materials that are relatively inexpensive and have excellent thermal conductivity. Although these materials have a high thermal conductivity (500 W/(mK) or more) in the direction of a carbon fiber, they have a thermal conductivity as low as 40 W/(mK) or less in the direction perpendicular to the carbon fiber, and also show anisotropic thermal expansion coefficients (fiber direction: 0 ppm/K, direction perpendicular to the fiber: 14 ppm/K).
Japanese Patent Laid-Open Nos. 2004-165665 and 2004-22828 (Patent Documents 1 and 2) disclose composite materials of carbon fibers and metals. Each of the methods for producing metal-based carbon fiber composite materials disclosed therein is considered to be one in which a molten metal is incorporated into voids in a pre-molded carbon fiber by the application of pressure (a molten bath impregnation method). In particular, in neither of the aforementioned documents, a nanofiber is not incorporated in the matrix, and it is difficult to disperse a nanofiber in a metal using the methods disclosed therein (a nanofiber reacts with molten aluminum and molten magnesium, and the nanofiber does not mix with molten copper due to poor wettability). Moreover, WO 2005/059194 (Patent Document 3) discloses a method for producing a metal-based carbon fiber composite material, whereby the formation of metal carbide is prevented, and a metal-based carbon fiber composite material that is lightweight, has a high thermal conductivity, and is capable of controlling the direction of heat flow is produced. This method comprises the steps of physically mixing a carbon fiber and a metal powder to obtain a metal-fiber mixture; charging the metal-fiber mixture into a jig while orienting the metal-fiber mixture; and placing the jig in air, vacuum, or an inert atmosphere, directly passing a pulsed current through the jig while applying pressure, and sintering the mixture with the heat thereby generated. In this publication, the metal-fiber mixture is oriented in one direction, and there is no disclosure concerning the thermal properties of the mixture such as a thermal expansion coefficient.    Patent document 1: Japanese Patent Laid-Open No. 2004-165665    Patent document 2: Japanese Patent Laid-Open No. 2004-2282B    Patent document 3: WO 2005/059194    Non-Patent Document 1: Furukawa Review No. 106, http://www.furukawa.co.jp/jiho/fj106/fj106—01.pdf